A solution for fixed side/corner box with only HTML/CSS.

A fixed side or corner box.markdown

A fixed side/corner box

Opened with :hover.

See the result of this example on CodePen.

fixed-side-corner-box.html

<div id="box">
  <h1>NEWSLETTER</h1>
  <div id="content">
    <p>Subscribe below to receive our news</p>
    <input type="email" placeholder="your e-mail" />
    <button type="submit">OK</button>
  </div>
</div>

style.css

* { border: 0; margin: 0; padding: 0; }

/* style */
body { background: #222; }

input {
  padding: 5px;
  background: #fff;
  width: 180px;
}

button {
  background: #115582;
  color: #fff;
  padding: 4px;
}
/* end - style */

#box {
  width: 300px;
  height: 100px;
  position: fixed;
  right: -278px; /* hidden */
  top: 60px;
  /* style */
  background: #0A2538;
  transition: all 0.7s ease;
  color: #fff;
  padding: 15px;
}

#box:hover { right: 0; }

#content {
  float: right;
  width: 240px;
  /* style */
  margin-top: 7px;
}

p {
  margin-bottom: 12px;
  /* style */
  text-align: center;
  line-height: 22px;
}

h1 {
  float: left;
  width: 28px;
  transform: rotate(-90deg);
  margin: 75px 0;
  /* style */
  font-size: 13px;
  font-weight: bold;
}